Most engines require a mixture enrichment system to improve or even allow starting. An enrichment system is especially necessary when the engine is cold, or after the engine has been run out of fuel. The two common means of enrichment in the field of small engines are the choke and the primer.
The choke provides enrichment by closing off the front end of the air intake of a carburetor to allow a manifold vacuum to draw on all of the fuel passages that open to the air intake. The disadvantage of the choke is that the user must close the choke completely and then crank the engine until they hear a false start. The choke is then partially opened and the engine is cranked until it starts. When the engine is warmed up 30 to 40 seconds, the choke is opened fully. This operation is too complex for many users and results in many field returns and complaints.
The primer uses a manually operated bulb to inject fuel into the carburetor throat. The disadvantage of the primer is that a simple primer cannot regulate the amount of fuel injected into the carburetor throat which makes it easy for the user to over- or under-prime the engine, resulting in no start.
An improved enrichment system uses an auxiliary fuel pump coupled with the carburetor body. The auxiliary fuel pump provides enrichment by injecting fuel into the carburetor throat when starting the engine. To prevent over- or under-priming the engine, the auxiliary-fuel pump uses a pulse emitted from the engine's crankcase to control the fuel injection. This approach is described in more detail in U.S. Pat. No. 5,704,774, entitled “Carburetor Start Pump Circuit,” filed on May 24, 1996, which is hereby incorporated by reference in its entirety.
When using a system such as the auxiliary fuel pump described above, it may be desirable to deactivate the system when the engine warms up because a constant fuel-air ratio will have been obtained, and thus, there would be no need for additional fuel injection. However, often times, the user may forget to do so or may engage the system when the engine is already running and is warmed up or hot. This may result in the engine “conking out” or stalling due to too much fuel being injected into the carburetor throat while the engine is running.
Therefore, it is believed that an improved carburetor start pump circuit would be desirable.